The Attorney General’s Open Government Internet Manual provides guidance on calculating the time to respond to a public records request:
1.6 E. An agency must respond to a request for public records within five business days of receipt of the request pursuant to RCW 42.56.520. Under RCW 1.12.040, the time allowed excludes the day of receipt from the computation. The agency ‘s initial response must either produce the record, or acknowledge receipt of the request and give a reasonable estimate of the time needed for response, or deny the request, or seek clarification of the requested records. http://www.atg.wa.gov/OpenGovernment/InternetManual/Chapter1.aspx
The Public Records Act only requires a response in five business days. A response letter mailed within five business days is in compliance with the law even if it is not received until after the fifth business day.